Absolutely agree, you've got a record of working out, shouldn't be a problem.

Like you I'd been working out regularly but not biking. (and a year previously had been completely sedentary, and I mean completely sedentary for several years). First couple of weeks I was doing 5 - 7 mile rides. Remember when I pushed myself to 10 miles, three weeks later I did 29 miles in just over 2 hours. In between rode 3 -4 times a week, inching up by a few miles each time - either doing another loop through the subdivision, going slightly further before turning back. You'll get a sense quickly to how far you can go, and how much further you can push it.

A well fitted bike makes a HUGE difference. As does a pair of padded biking shorts/liner.

And if you register for the longer ride, couldn't you always back down to the shorter distance if you realize you need a bit more time to train?